In conventional laser scanning, feature deformations such as distortion and skew are very common. There are three kinds of distortion for example, any of which may be present in an optical unit: pillow-shaped distortion, in which magnification increases with distance from the axis as shown in FIG. 1A; barrel distortion, in which magnification decreases with distance from the axis as shown in FIG. 1B; and barrel-pillow-shaped distortion, being the combination of the pillow-shaped and the barrel-shaped deformation as shown in FIG. 1C. Thus, it is required to perform a calibration process upon the laser machining apparatus for compensating such errors.
As we see currently in the industries, the laser machining errors are usually being calibrated and adjusted by a manual operation. Please refer to FIG. 2A and FIG. 2B, which schematic diagrams respectively showing a conventional laser calibration strip having a square pattern of 12×12 dot matrix formed thereon and showing the conventional calibration strip of FIG. 2A being laser machined and thus having a distorted pattern formed thereon. As shown in FIG. 2A and FIG. 2B that the calibration pattern 71 formed on the conventional calibration strip 70 is a square pattern of 12×12 dot matrix and the distorted pattern 72 is a pillow-shaped distortion, it is possible to measure the laser machining error between the distorted pattern 72 and the calibration pattern 71 manually by the use of a measurement tool such as a ruler. Please refer to FIG. 3, which is a flow chart depicting steps for compensating the laser machining error. In FIG. 3, the flow starts at step 91, in which a calibration table is generated according the a manual measurement operation shown in FIG. 2B and then the so-generated calibration table is fed to a conversion program to be converted; and then the flow proceeds to step 92. At step 92, a calibration file recognizable by a control card is generated from the conversion of the conversion program that is transmitted to the control card; and then the flow proceeds to step 93. At step 93, the control card is going to perform a distortion compensation process according to the calibration file.
However, the aforesaid method for calibrating laser machining error has the following shortcomings:                (1) As a calibration strip made of a specific material is only suitable for calibrating a laser of a specific wavelength, various calibration strips made of different materials are required.        (2) For facilitating the manual measurement, it is preferred to use a laser of larger power to form a more distinguishable distorted pattern. However, the large-powered laser can inflict more severe thermal deformation upon the calibration strip and thus adversely affected the accuracy of the measurement.        (3) The manual measurement and calibration is a time consuming work if there are too many dots in the dot matrix of the calibration pattern, e.g. when there are more than 256 dots existed in the calibration pattern.        
There are already many studies trying to improve the aforesaid shortcomings. One of which is disclosed in U.S. Pat. No. 6,501,061, entitled “Laser calibration apparatus and method”, which shows a system for positioning a focused laser beam over a processing area with high precision by the detection of a charge coupled device (CCD). It is an on-line calibration method that is basically performed by the use of: a laser scanner having scanner position coordinates for scanning the focused laser beam over a region of interest on a work surface; a CCD for detecting when the focused laser beam is received at the work surface. As a specific CCD can only detects laser beams of wavelength in a specific range, the aforesaid apparatus must be provided with various CCDs so as to be used for detecting laser beams ranged from 248 nm to 10.6 μm. It is noted that the aforesaid apparatus can be very costly especially when a CCD for detecting laser beam in an invisible wavelength range is required, as such CCD can be 5 times to 10 times more expensive than other common CCDs. Moreover, the energy of the laser beams used in the aforesaid apparatus must be decayed before it is detected by the CCD.
As in many laser processing applications, it is necessary to position a focused laser beam over a processing area with very high precision. Therefore, a rapid and accurate on-line laser calibration apparatus is becoming a necessity for mass production.